PROJECT SUMMARY The oral mucosal microbiota is a complex ecosystem primarily represented by bacteria and fungi. Most oropharyngeal fungal infections are caused by the genus Candida and are assumed to result from an overgrowth of indigenous species, primarily C. albicans. C. albicans is a commensal colonizer of the oral mucosa in humans, but is also responsible for infections afflicting immunocompromised hosts. Persistent oropharyngeal thrush is refractory to most antifungals and a significant clinical problem in pharmacologically immunosuppressed patients. Corticosteroid-induced and chemotherapy-induced immunosuppression, are two main risk factors for oropharyngeal candidiasis in humans. C. albicans also causes fungemia, a serious consequence of cancer cytotoxic chemotherapy, which is thought to develop from fungal translocation through compromised mucosal barriers. Changes in endogenous bacterial population size or composition and in the host environment can transform fungal commensals into pathobionts. Work in our previous funding cycle established a synergistic relationship of mitis group streptococci with C. albicans in the pathogenesis of oral candidiasis. We identified mechanisms of synergy which involved both a direct effect on fungal virulence gene expression and a modification of host responses. In this project we will build on our ongoing studies examining the interplay of the resident oral mucosal bacterial microbiota and C. albicans. We will use mouse models of commensal colonization or mucosal infection to interrogate oral bacterial microbiome parameters that promote C. albicans virulence. In aim 1 we will characterize dysbiotic changes in mucosa-associated bacterial communities in oropharyngeal candidiasis, using our established mouse models of cortisone- and chemotherapy-induced immunosuppression. We will then test the hypothesis that certain endogenous bacterial species isolated from dysbiotic states can exhibit pathogenic synergy with C. albicans. In aim 2 we will define the regulatory mechanisms of fungal-bacterial mucosal biofilm growth in each immunosuppression state. Finally, in aim 3 we will examine the role of the dysbiotic communities and host response in mucosal barrier breach and bloodstream dissemination by C. albicans. The proposed studies have the potential to lead to a paradigm shift in how clinicians and scientists view the microbiome changes characterizing mucosal Candida infections. This project will identify certain oral bacteria as new, clinically relevant mediators of invasive fungal infections thus providing justification for the combined use of antifungal and anti-bacterial treatments in at risk patients. A better understanding of the relationship between fungi and the oral microbiome could also result in new biomarkers of infection risk or identification of probiotic commensals that could lower the likelihood of invasive mucosal candidiasis in high-risk populations such as patients undergoing intensive cancer chemotherapy.